Metal beverage cans are designed and manufactured to withstand high internal pressure—typically 90 or 100 psi. Can bodies are commonly formed from a metal blank that is first drawn into a cup. The bottom of the cup is formed into a dome and a standing ring, and the sides of the cup are ironed to a desired can wall thickness and height. After the can is filled, a can end is placed onto the open can end and affixed with a seaming process.
It has been the conventional practice to reduce the diameter at the top of the can to reduce the weight of the can end in a process referred to as necking. Cans may be necked in a “spin necking” process in which cans are rotated with rollers that reduce the diameter of the neck. Most cans are necked in a “die necking” process in which cans are longitudinally pushed into dies to gently reduce the neck diameter over several stages. For example, reducing the diameter of a can neck from a conventional body diameter of 2 11/16th inches to 2 6/16th inches (that is, from a 211 to a 206 size) often requires multiple stages, often 14.
Each of the necking stages typically includes a main turret shaft that carries a starwheel for holding the can bodies, a die assembly that includes the tooling for reducing the diameter of the open end of the can, and a pusher ram to push the can into the die tooling. Each necking stage also typically includes a transfer starwheel to transfer cans between turret starwheels. Often, a waxer station is positioned at the inlet of the necking stages, and a bottom reforming station, a flanging station and a light testing station are positioned at the outlet of the necking stages.
The collective stages of the can necking process, including the various components described above may collectively be referred to as a can necking machine or a multi-stage can necking machine. In a properly operated can line, cans fill the pockets of the necking machine in an unbroken, serpentine line. In part because of the high speed operation of can necking machines, however, errors may occur during the can necking process. One type of error may be evidenced by losing cans from a can necking machine (that is, a pocket that should have a can does not have a can). A can lost from the can necking machine may also be referred to as a “dropped” can, and encompasses a can that enters the can necking machine but is not properly retained and a pocket that lacks a can because of a can feed error (that is, the line of cans is broken because of a break in the continuous can feed).
Identifying can drop rates may assist in troubleshooting a can necking machine. However, increasing the number of stages or increasing the speed of the can necking process may make timely identification of can drop rates difficult or limit the speed at which a can necking machine may be operated.